Cathodic electrocoat compositions are very widely used in many industrial coating processes. The compositions provide high paint utilization rates, low environmental contamination and excellent corrosion resistance to metal substrates. Although surface coatings of excellent corrosion resistance can be achieved by the use of cathodic electrocoat compositions, a problem associated with this type of coating is the development of surface defects upon curing, particularly craters. Typically, impurities carried into the electrocoating bath with the substrate to be coated can cause such surface defects. Examples of such impurities can include, for example, particulates or incompatible oils, such as, for example, lubricating oil, anti-corrosion grease and joint sealing compounds.
A number of anticrater additives are known that can help to reduce such surface defects. For example, various water reducible polyesters, silane modified polyether amines, and polyvinyl ethers have been developed for use as anticrater additives. U.S. Pat. No. 6,849,169 describes the use of homopolymers or copolymers of 2-ethyl hexyl acrylate as an additive in aqueous cathodically depositable coatings to suppress the formation of surface defects in coating films.
Chung et al U.S. Pat. No. 5,356,960 issued Oct. 18, 1994 shows an anticrater additive that forms a crater free, smooth and even finish. However, when this additive is used in an electrocoating composition that is subsequently baked in an indirect gas-fired oven, the anticrater additive migrates very readily to the surface of the electrocoating composition during baking. Any primer compositions applied over such a surface containing polymeric melamine crosslinking agents adhere poorly to the electrocoat composition and hence, adhesion failure of any topcoat applied over the primer is readily evident.
An anticrater additive is needed that will not migrate to the surface of the deposited electrocoating composition during baking and should not adversely affect other properties such as the throwing power of the electrocoating bath, the curing of the deposited coating or the film properties of the resulting finish. There is a continuing need to improve the performance and adhesion characteristics of anticrater additives.